A person who believes their metal music is superior to other people's music. These people only listen to metal music and categorize metal through sub-genres. Some metal elitists are so wound up, that they tend to only listen to one type of metal sub-genre. Metal Elitists are usually found listening to underground metal bands and cut down other people's musical taste if it is not up to par with their own.

"Dude that guy was an asshole. He completely cut down my music and told me that I was wrong for listening to the music that I do, saying it wasn't "real" metal and then he went on to call me a hot topic mallcore." - Person A

A person who hates most metal that is popular, and would rather listen to some shitty metal band with a name nobody would care to pronounce correctly and a fan base of 5 people in the whole world. Anymore is considered too popular for their liking and is automatically deemed a shit band. They will flame the 13 year olds who mistake nu-metal bands for actual metal.

More formally, a 'Heavy Metal Elitists', are members of the Heavy Metal community/fan-base, usually aficionado, whose long term experiences with Metal music has often led them to develop a sense of passion that is frequently interpreted as stubbornness or arrogance.
Often times their knowledge of heavy metal not just limited to knowing lots of bands. Most are familiar with the musical characteristics (scales, theories, etc.), attitudes, history, people, and social issues relating to heavy metal.
Usually Elitists shun people new to metal because the noobs are unable to cite knowledge from those aforementioned topics to support their statements about a band.
One misconception is that they do not listen to any other music besides Heavy Metal and some will even restrict themselves to only particular metal sub-genres. While it is common practice to adhere to only a few sub-genres within their taste of Heavy Metal, however outside of Metal, many do have diverse tastes in music. Although they may detest mainstream music like pop or hip-hop, most are often fond of other music such as jazz, blues, classical, and punk, since these types of music are highly influential to Heavy Metal.
Another misconception is that they think Heavy Metal is the most superior music on earth. Most Elitists will admit that the skills and novelty in the some compositions of jazz and classical marvel that of Metal.

Do not confuse a Metal Elitist with:
1) an asshole who happens to like metal, or 2) a metalhead who happens to be an asshole. Some Elitist, by nature, will be an asshole (in turn) if you make unsupported statements about certain bands.

A rude metalhead who tells people they "aren't a real metalhead" or a calls them a "poser" because they dont like/listen that particular sub-gerne of metal. They put others down for their musical taste or if they don't like the same bands as them, they think they know what "real metal" is.

Typical Metal Elitist : Hahahaha i'm such a badass metalhead! i know all there is to know about everything all your shitty music can never stand up to my music, since i'm the only 'true metalhead' on the face of the earth. Ha! Beat that posers! hahaha!

A douchebag who insists that any band you mention that is not Black Metal, Death Metal, Folk Metal or Power Metal, sucks ass and is not metal at all. They'll often bombard said band with weird and immature insults such as: "They make me want to eat my own cock and vomit."
Bands they prefer are almost always obscure and unknown to anybody and from Scandinavia. They hate all bands that are popular. The only Thrash Metal band that is tolerated by Metal Elitist is Iron Maiden. The rest isn't even metal.

Metal Elitist: Oh my god, your list sucks bloody cocks. Shit isn't even metal. Weird Al Yankadick can make better metal than Crobar. Besides, all that shit is pop-rock, you fucking twit. I'd rather have a tumor in my scrotum and guiche than to listen to Wolber's crying again.

A person who thinks very highly about his own particular taste in heavy metal, and pretty much cans the rest of it. Sometimes they go as far as denying some bands as metal when they clearly are. They always act like they're higher than other people, when they themselves act like children when it comes to music. As others have said, they often make over-exaggerated critiques of metal bands and would rather experience a form of physical torture or genital mutilation than to listen to some metal bands. They are never constructive, polite, merciful or mature when it comes to criticism.

Metal Elitist: Wow, you listen to Immortal? They're fucking gay and sound like a garbage disposal full of spoons. I'd rather shove a knife up my ass than to listen to this cheerleader piece of shit. I recommend you go listen to some Gaarfadinonskelyrgen they're 10x better.

In short, the term is generally used to describe someone who thinks they are better than everyone else because of their taste in music. They tend to only listen to metal bands that are very obscure and will stop listening to said bands if they find out that more than 10 people listen to them, so they can be "unique". If they do happen to like a band that has become well known, (e.g Metallica) they will state that they only like their demos or first album and that everything else is crap. They usually bitch about bands like Cannibal Corpse for sounding the same on all of their records but complain just as much when a band happens to experiment or change their style on their next album (e.g Carcass, Morbid Angel.) Isn't that funny?